How to Append Text to a File in Go

Created May 6, 2022 at 13:30

Modified May 6, 2022 at 13:40

Using os.OpenFile Function

OpenFile is the generalized open call; most users will use Open or Create instead. It opens the named file with specified flag (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag is passed, it is created with mode perm (before umask). If successful, methods on the returned File can be used for I/O.

See the following example:

package main

import "os"

func main() {

  // If the file doesn't exist, create it, or append to the file
  f, err := os.OpenFile("access.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
  if err != nil {
    // log.Fatal(err)
  }
  defer f.Close()

  if _, err := f.Write([]byte("appended ...\n")); err != nil {
    // log.Fatal(err)
  }
}

appended ...
appended ...

If the file doesn't exist, create it, or append to the file.

Constants:

const (
  // Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
  O_RDONLY int = syscall.O_RDONLY // open the file read-only.
  O_WRONLY int = syscall.O_WRONLY // open the file write-only.
  O_RDWR   int = syscall.O_RDWR   // open the file read-write.
  // The remaining values may be or'ed in to control behavior.
  O_APPEND int = syscall.O_APPEND // append data to the file when writing.
  O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
  O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
  O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
  O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
)